1. Field of the Invention
This invention relates to method of controlling an inverter for pulse-width modulating (hereinafter referred to as "PWM") a voltage source inverter for driving a three phase AC load and system therefor.
2. Description of the Prior Art
FIG. 1 is a circuit diagram theoretically showing the arrangement in which an AC motor i.e. induction motor (IM) is driven by a voltage source inverter 3 which is connected between a DC power source 1 outputting a DC voltage Vd and the AC motor 2, and the voltage source inverter 3 switches semiconductor switches 31, 32 and 33 equal in number to the load phases at a predetermined cycle to thereby obtain three phase alternating currents.
The voltage source inverter 3 having the above-described arrangement can take a potential of anode or cathode of the DC power source 1. Then, there has generally been practiced such a PWM control that, in order to control the speed of the AC motor in a wide range, a multiplicity of notches are applied to a square output waveform, and the positions and widths of these notches are controlled, whereby an output voltage waveform and its effective value are controlled. The final object of this control resides in that a magnetic flux as being a vectorial value connected in interlinkage to a rotor of a motor is caused to make a uniform rotary motion at a desired scale and a desired rotary speed. With this arrangement, a higher harmonic current flowing to the motor can be removed to a considerable extent, so that the iron loss and copper loss due to the harmonic contents of the waveform can be decreased and the torque pulsation can be reduced.
Now, as a method of conducting the PWM control, there has heretofore been used a triangle interception method. According to this method, mean values with time of the respective phase potentials are approximated to sinusoidal waves. To state specifically, there is obtained as an inverter a rectangular wave signal having as its on-off cycle the intersections between a desired sinusoidal wave and a triangular wave signal having a predetermined cycle.
As for the maximum amplitudes of sinusoidal waves which can be approximated to the phase potentials in the above-described control, phase potentials va and vb of two phases out of the three phases may be shown as in FIGS. 2(a) and 2(b). Each of the values is one half the DC voltage Vd, and the maximum amplitude vab of the line voltage is extracted through the following equation. ##EQU1##
The maximum amplitude vab is an amplitude of 3/2 the DC voltage Vd as shown in FIG. 2(c).
However, if the line voltage can be directly approximated to the sinusoidal wave, then the line voltage vab can output as high as the DC voltage Vd as the maximum amplitude as apparent from FIG. 2(c). However, since the line voltage is determined by the difference between the respective phase potentials, each of the line voltages cannot be singly and simultaneously approximately to the sinusoidal wave. For example, if the a phase potential va and the b phase potential vb are controlled so as to approximate the line voltage vab to the sine wave, then the line voltages vbc and vca are subjected to the influence of the control, whereby the line voltages vbc and vca cannot be approximated to the sinusoidal wave. In consequence, when the triangle interception method is applied, there is no other way than that the phase voltages are turned into the sinusoidal waves so as to indirectly approximate the line voltages to the sinusoidal waves, the maximum amplitude of each of the line voltages can reach 3/2 the original capacity. Therefore, the maximum output of the inverter becomes a value of 3/2 the maximum value of the original output. When a DC power source having a voltage of 2/3 times is prepared in advance in order to control the decrease of the maximum output, such disadvantages are presented that the withstanding voltage of elements used are increased, the capacity of the inverter is increased and the manufacturing cost is raised. Furthermore, since the power source voltage is on-off controlled to be turned into an output voltage for controlling the magnetic flux on the assumption that there is no ripple according to the triangle interception method, there has been presented such a disadvantage that, a ripple, if any, in the DC voltage Vd becomes a ripple in an output voltage and affects the control of the magnetic flux. Because of this, it has become necessary to provide a smoothing circuit of a high capacity on an input section, which proves to be impractical.
Except the above-described method, there is PWM control method in which the primary current is approximated to the sinusoidal wave. However, according to this PWM method, an induction motor uses iron as the core, and there is shown the hysteresis characteristics where the exciting current and the magnetic flux are not in a linear relationship due to non-linear properties of iron as shown in FIG. 3, whereby, even if the primary current is turned into the sinusoidal wave, the magnetic flux .lambda..sub.p connected in interlinkage to the motor rotor does not make a uniform rotary motion but a crooked circular motion. Furthermore, according to this method, in order to turn the currents of the respective phases into the sinusoidal waves, the potentials of the respective phases are changed over separately of each other, whereby, to tell in detail, the rotary motion of the magnetic flux consists of a series of modes composed of rotations in the normal direction, rotations in the reverse direction and stops, so that useless loops are formed, thus resulting in increased iron loss of the motor. Additionally, when the induction motor is driven through the control of the primary current, there is presented the disadvantage that, because the impedance of the induction motor is greatly varied due to the slip, the speed control cannot be easily conducted with the arrangement in which no feedback to the rotor speed is made.
The drawback of the control method of the prior art as described above resides in the attempt of turning the voltage or current into the sinusoidal wave so as to indirectly cause the magnetic flux to make a uniform rotary motion.